Major depressive disorder (MDD) afflicts about 9.5% of the U.S. population over the age 18. There is an urgent need for a novel, more effective therapeutic strategy for MDD treatment, as less than 50% of depressed patients achieve full remission and many individuals are not responsive to currently available monoamine-based traditional antidepressants. While there has been over 50 years of effort made to understand the neural mechanism underlying this disease, current therapeutics have not drastically changed from the monoamine hypothesis. Recent studies have shown Deep Brain Stimulation (DBS) to be robustly efficacious in treating MDD patients and thus research has focused on discovering the underlying mechanisms of this therapy. In this way, a new paradigm has emerged with MDD being viewed as a neural circuit disorder. Therefore, it is crucial to understand the neural circuits underlying MDD, and more importantly, the specific projection pathways implicated in this disease. In recent years, an increasing body of evidence shows that depression-like behaviors such as social avoidance and anhedonia are associated with altered activity of ventral tegmental (VTA) dopamine (DA) neurons that project to three emotion-related brain regions: the nucleus accumbens (NAc), medial prefrontal cortex (mPFC) and amygdala. In a chronic social defeat mouse model of depression, Dr. Han and colleagues previously found that the activity of VTA DA neurons in the brain reward circuit is a key determinant of susceptibility vs. resilience to social defeat stress. The firing rate and bursting events of these neurons was significantly increased by chronic defeat in susceptible mice, but not in resilient subgroup, a subpopulation of mice that went through chronic social defeat, but do not show depression-like behaviors. Furthermore, an experimentally (virally or optogenetically) induced increase in the firing rate and burst firing of projection-mixed VTA DA neurons promoted a susceptible phenotype, while a decrease in the firing rate promoted resilience. Interestingly, the level of BDNF is upregulated in the VTA target area NAc only in susceptible mice. On the basis of these and other previous studies, this proposal hypothesizes that chronic social defeat induces an increase in VTA DA firing rate and bursting mechanisms specifically in VTA DA neurons projecting to the NAc, in susceptible mice. Furthermore, it is hypothesized that the behavioral phenotypes of susceptibility and resilience are determined by pathway specific dopamine neurons and specific firing patterns. This proposal seeks to intensively characterize the projection specific VTA DA neurons to the NAc, mPFC, and amygdala through the use of viral-mediated gene transfer, electrophysiological and optogenetic techniques. These proposed molecular, cellular and behavioral studies will provide very useful and highly novel information both for improving our knowledge of the circuitry of depression and for identifying new therapeutic circuitry targets to develop more effective treatments for depression.